Hitherto, on account of its low aggressiveness to patients, medical treatment has been performed by operations, for example, such as percutaneous transluminal coronary angioplasty (PTCA) and the like to a stenosed part of the coronary artery for angina pectoris, myocardial infarction and the like and by operations, for example such as percutaneous transluminal angioplasty (PTA) and the like to a stenosed part for ischemic disease and the like of the femoral artery and the carotid artery.
These methods of treatment are techniques to secure and resume the flowing of blood by widening a stenosed or occluded blood vessel using a catheter equipped with a balloon which is compactly folded at a distal end of the catheter.
In order to help prevent the widened blood vessels from becoming stenosed or occluded again, a treatment of placing a metallic stent in a blood vessel and securing patency of the blood vessel has been performed.
Generally, a stent is cut from a metallic pipe or has a mesh shape, a coil shape or the like made of a metallic wire. However, every stent has a tubular structure of which a diameter can be decreased, and is inserted into a blood vessel through a catheter while having its diameter in a decreased state, thereby expanding in diameter and being placed in a stenosed part so as to mechanically support a lumen in the blood vessel.
There is a stent in which a radiopaque metallic material is provided at the end portion of the stent as a marker so as to be able to favorably confirm a position of the stent through radioscopy.
Incidentally, each metal has a unique electric potential.
When metals of different types are brought into contact with each other and are immersed in an electrolyte solution, a potential difference (galvanic potential) can be generated between a less-noble metal (metal having greater ionization tendency) and a noble metal (metal having smaller ionization tendency) due to a difference in electric potential therebetween so that a battery (local battery, galvanic battery) is formed, and a current (local current) flows, thereby causing corrosion.
Such corrosion which is caused by an electrochemical reaction resulting from forming of the local battery having the dissimilar metals as electrodes can be referred to as galvanic corrosion, dissimilar metal contact corrosion or local current corrosion.
As a metallic material constituting a stent, stainless steel (for example, SS316L), a cobalt-based alloy, a nickel-titanium-based alloy or the like is generally used.
As a radiopaque marker, gold or platinum having a highly radiopaque property is generally used. Gold and platinum have extremely high electric potentials.
For this reason, the potential difference is increased between the metallic material used in the stent and gold or platinum used in the radiopaque marker causing the galvanic corrosion described above, thereby resulting in liquation of the less-noble metal.
Therefore, when the stent is placed in a living body for a long period, there is a possibility that the metal may liquate in a body fluid such as blood, thereby causing a safety problem.
In consideration of the above-described problem, JP-A-2002-263195 (corresponding to U.S. Patent Application Publication No. 2001/0001317) discloses that a radiopaque tab is provided at a stent by micro-alloying the radiopaque metallic material such as gold or platinum with a nickel-titanium-based alloy constituting the stent (See, for example, paragraph [0038] in JP-A-2002-263195).
According to the disclosure, there is no remarkable occurrence of a galvanic element (See, for example, paragraph [0016] in JP-A-2002-263195).
For example, according to JP-A-2002-263195 described above (corresponding to U.S. Patent Application Publication No. 2001/0001317), an infinitesimally small amount of radiopaque metallic material can be added to a metallic material (for example, nickel-titanium-based alloy) constituting a stent and as such a sufficient radiopaque property cannot be applied to the stent.
Therefore, when performing placement of the stent, a position of the stent cannot be favorably and visually confirmed through radioscopy.
Meanwhile, if the amount of radiopaque metallic material to be added to the metallic material constituting the stent is increased in order to enhance the radiopaque property, it is no longer in a micro-alloyed state causing frequent occurrences of phase separation or extraction. As a result, a potential difference (galvanic potential) can be generated between the metallic material configuring the stent and the radiopaque metallic material, thereby accelerating galvanic corrosion.